The expansion of an underground mine (e.g. a coal mine) requires digging a tunnel which initially has an unsupported roof. To stabilize and support the roof a roof bolt must be inserted into the roof to provide support. The operator must first drill holes in the roof through the use of a rotatable cutting bit or roof drill bit. A roof bolt is then inserted into each one of the holes.
A common roof drill bit design uses a cutting insert that has been brazed into a slot at the axially forward end of the roof drill bit body. U.S. Pat. No. 5,400,861 to Sheirer discloses various roof drill bits. U.S. Pat. No. 4,603,751 Erickson also discloses various roof drill bits. Applicants hereby incorporate U.S. Pat. Nos. 4,603,751 and 5,400,861 by reference herein. In addition, the following catalogs published by Kennametal Inc. of Latrobe, Pennsylvania (U.S.A.), which are hereby incorporated by reference herein, disclose roof drill bits: "Kennametal Mining Products", Catalog A96-55(15)H6 (September 1996) [36 pages in length], and "Kennametal Mining Products" Catalog B92-75R(3)M5 (1992) [36 pages in length].
While brazed-on cutting inserts have provided adequate results in the drilling of holes, there have been some drawbacks associated with the utilization of the brazed-on cutting inserts. As a result of brazing, the difference in the coefficients of thermal expansion between the steel roof drill bit body and the cemented carbide (e.g., tungsten carbide-cobalt alloy) cutting insert has caused residual stresses in the cemented carbide cutting insert. These residual stresses have been detrimental to the performance of the roof drill bit since they have lead to premature failure of the cutting insert. This has been especially true in those cases where the earth strata being drilled has resulted in high impact loading on the cutting insert.
The presence of these residual stresses also has required that the grades of cemented carbide used for the cutting insert have had a high transverse rupture strength. This has been a factor which has limited the number of grades which have been suitable candidates for a cutting insert in a rotatable cutting bit such as a roof drill bit.
Some materials (e.g., ceramics, low binder content [3 to 6 weight percent binder] tungsten carbide, binderless tungsten carbide, diamond or refractory [CVD or PVD] coated cemented carbides or ceramics, polycrystalline diamond [PCD] composites, polycrystalline cubic boron nitride [PcBN] composites) may have been suitable materials for use as a cutting insert in a roof drill bit because of their increased wear resistance, but have not been good candidates for use as a cutting insert in a roof drill bit due to brazing difficulties. More specifically, either these materials have been difficult to satisfactorily braze, or when brazed, these materials have experienced unacceptably high residual brazing-induced stresses.
In view of the drawbacks associated with brazing the cutting insert into the seat of a roof drill bit, it would be desirable to provide a roof drill bit wherein the cutting insert would be affixed within the seat of the roof drill bit without using a brazing process. Such a roof drill bit would have less of a chance of premature failure due to the presence of residual stresses. Such a roof drill bit would be able to use a wider range of materials for the cutting insert than has been heretofore available.
There comes a point where the cutting insert in the roof drill bit has reached a condition where the cutting action by the bit is no longer sufficient. At this point one of two processes occurs. One process comprises the regrinding of the cutting insert without removing the cutting insert from the roof drill bit. The other process comprises debrazing the cutting insert so as to be able to remove it from the roof drill bit body, and then brazing a new cutting insert to the roof drill bit body. Each process has certain costs associated therewith which add to the overall cost of the drilling operation.
To reduce these additional costs it would be desirable to provide a roof drilling bit which would not require regrinding to place the cutting insert back in condition for cutting. It would also be desirable to provide a roof drilling bit that does not require debrazing/brazing of the cutting insert to replace a worn cutting insert.
Roof drill bits which have a higher penetration rate for the drilling operation are desirable in that such a drill typically takes less time to drill the required number of holes in the mine roof (i.e., earth strata). The ability of the roof drill bit to use a cutting insert made from a more wear resistant material, such as those identified above, enhances the potential to maintain a higher penetration rate at a given thrust level for a longer time. Thus, it would also be desirable to provide an improved roof drill bit that has a high penetration rate.